Virtualisation for Beginners

It's not just for server jockeys, you know

This is something that Windows itself has always purported to be able to do since the arrival of XP, but - in my experience at least - the success rate has been less than sterling. A VM snapshot, by contrast, is equivalent to a complete new installation on brand new hardware with a slipstream update of all your apps up to the last point of failure.

Technically, if perhaps not legally, you can run Mac OS X in a VMWare virtual machine under Windows

The Technology Behind the Magic

Virtualisation originated in IBM's big mainframes of the 1960s and 1970s when the company hit on the idea of installing a thin software layer, the 'hypervisor' immediately above the hardware that would allow multiple operating systems to run simultaneously side by side without impacting on one another or even necessarily being aware of each others presence.

'Simultaneously' needs some qualification here, because the various operating systems in fact time-shared a single CPU. But the switching would take place fast enough - several thousand times a second - to create the illusion of simultaneity.

Multi-tasking operating systems use a similar technique to run multiple apps at the same time. In this scenario, called 'context switching', only the metrics associated with each running app - the 'state' of the app - need be stored. When switching between complete guest operating systems much more data has to be saved between each switch: the total state of the guest operating system itself, as well as the states of all the apps it happens to be running at the time. Engineers call this a 'world switch'.

A Solid Proposition

The earlier Pentium processors weren't designed with virtualisation in mind. But using a software technique called 'segment faulting', developers managed to introduce world switching, and hence virtualisation to Pentium-class processors in the late 1990s. The first VMware product allowed Windows to run in a virtual machine on a Linux host, and this technology was later extended to use Windows as a host for Linux and other operating systems.

Parallels 5 for the Mac allows you to set an 'Active Corner' when in Full Screen mode

By 2006, even modest desktop processors from Intel and AMD were becoming powerful enough for virtualisation to be a solid proposition. And that, literally, is what it became. The two x86 companies each introduced hardware assistance for virtualisation into its processors: AMD-v - codenamed 'Pacifica' - and Intel's own VTx - 'Vanderpool' - were similar but different hardware virtualisation support systems.